ID_TOKEN got from https://appleid.apple.com/auth/tokenendpoint should contain firstName and lastName for the user in the same way as it currently contains the email field
This is by design. From https://developer.apple.com/documentation/signinwithapplerestapi/authenticating_users_with_sign_in_with_apple
"If you request the user’s full name, Sign in with Apple collects the information to pass along to your app. The name defaults to the user’s name from their Apple ID, but the user can change their name. The modified name is only shared with your app and not with Apple, and hence isn’t included in the ID token.”